Amino acids-Definition, Classification, Properties, and Mnemonic codes

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What are amino acids?

Amino acids are organic compounds that contain two functional groups— amino and carboxyl. The amino (—NH2) is basic while the carboxyl(—COOH) is acidic in nature.
Generally, they exist as dipolar ions commonly referred to as zwitterions.

How many amino acids are there?

20 amino acids are commonly found in proteins. Recently, a 21st amino acid namely selenocysteine has been added. Selenocysteine is found at the active sites of some enzymes/proteins (selenoproteins). e.g. glutathione peroxidase, glycine reductase, 5′-deiodinase, thioredoxin reductase. Selenocysteine contains the trace element selenium instead of the sulfur atom of cysteine.

What are essential amino acids?

Essential amino acids are those which cannot be synthesized by the body and, therefore, need to be supplied through the diet. They are essential for the proper growth and maintenance of the individual.

What are the examples of essential amino acids?

The ten amino acids are given below are essential for humans:
Arginine, Valine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan.
The two amino acids namely arginine and histidine are synthesized by adults and not by growing children, hence these are considered as semi-essential amino acids. Hence, 8 amino acids are absolutely essential while 2 are semi-essential.

Amino acids are linked together by which bond?

The amino acids are linked together by covalent peptide bonds. A peptide bond is formed when the amino group of an amino acid combines with the carboxyl group of another amino acid.

Which amino acids contain sulfur?

Cysteine with sulfhydryl group and methionine with thioether group are the 2 amino acids incorporated during the course of protein synthesis. Cystine, another important sulfur-containing amino acid, is made by condensation of two molecules of cysteine.

How many amino acids are found in protein?

Proteins are nitrogen-containing macromolecules that include L-α-amino acids as the repeating units. Half of the 20 amino acids found in proteins can be synthesized by the body (nonessential) while the rest have to be provided in the diet (essential amino acids).

Amino Acids Classification Based on the Structure

Amino acids with aliphatic side chains:

These are monoamine monocarboxylic acids and are the most simple amino acids—glycine, alanine, valine, leucine, and isoleucine. The last three amino acids (Leu, Ile, Val) contain branched aliphatic side chains; hence they’re mentioned as branched-chain amino acids.

Hydroxyl group-containing amino acids:

Serine, threonine, and tyrosine are examples of hydroxyl group-containing amino acids. Tyrosine is usually considered under aromatic amino acids.

Sulfur-containing amino acids:

Cysteine with sulfhydryl group and methionine with thioether group are the 2 amino acids incorporated during the course of protein synthesis. Cystine, another important sulfur-containing amino acid, is made by condensation of two molecules of cysteine.

Acidic amino acids and their amides:

Aspartic acid and glutamic acids are chemically dicarboxylic monoamine acids while asparagine and glutamine are their respective amide derivatives.

Basic amino acids:

Lysine, Arginine (with guanidino group), and Histidine (with imidazole ring) are chemically dibasic monocarboxylic acids. They are highly basic in character.

Aromatic amino acids:

Phenylalanine, tyrosine, and tryptophan (with indole ring) are aromatic amino acids. Additionally, histidine can also be considered under this category.

Imino acids:

Proline containing pyrrolidine ring is a unique amino acid. It has an imino group (=NH), instead of an amino group (–NH2) found in other amino acids. Therefore, proline is an α-imino acid.

Heterocyclic amino acids:

Histidine, tryptophan, and proline.

Amino Acids Functional Groups

The amino (—NH2) is basic while the carboxyl (—COOH) is acidic in nature.

Mnemonic for Essential Amino Acids

Essential Amino Acids: Arginine, Valine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan.

The mnemonic code for essential amino acids can be A.V. HILL, MP., T. T. (first letter of every amino acid) could also be memorized to recall essential amino acids.

Other useful codes are

H. VITTAL, LMP;

PH. VILLMA, TT,

PVT TIM HALL

MATTVILPhLy.

Ketogenic and Glucogenic Amino Acids

Amino acids can also be classified based on their metabolic fate. The carbon skeleton of amino acids can function as a precursor for the synthesis of glucose (glycogenic) or fat (ketogenic) or both.

  1. Glycogenic amino acids: These amino acids can function as precursors for the formation of glucose or glycogen. e.g. alanine, aspartate, glycine, methionine, etc.
  2. Ketogenic amino acids: Fat can be synthesized from these types of amino acids. Two amino acids leucine and lysine are examples of ketogenic amino acids.
  3. Glycogenic and ketogenic amino acids: The four amino acids isoleucine, phenylalanine, tryptophan, tyrosine are precursors for the synthesis of glucose and fat.

Amino Acids One Letter Code

Abbreviations of amino acids

Mnemonic of amino acids

Amino Acids Physical Properties

The amino acids differ in their physicochemical properties which in turn determines the characteristics of proteins.

1. Solubility: Many amino acids are usually soluble in water and insoluble in organic solvents.

2. Melting points: Amino acids generally melt at higher temperatures, often above 200°C.

3. Taste: Amino acids could also be sweet (Gly, Ala, Val), tasteless (Leu), or bitter (Arg, Ile). Monosodium glutamate (MSG; Ajinomoto) is used as a flavoring agent in the food industry, and Chinese foods to increase taste and flavor.

4. Optical properties: All amino acids except glycine possess optical isomers due to the presence of asymmetric carbon atoms. Some amino acids even have a second asymmetric carbon e.g. isoleucine, threonine.

5. Amino acids as ampholytes: Amino acids contain both acidic (Carboxylic –COOH) and basic (Amino –NH2) groups. They can donate a proton or accept a proton, hence amino acids are considered ampholytes.

Acid-Base Properties of Amino Acids

The amino acids rarely exist in a neutral form with free carboxylic (–COOH) and free amino (–NH2) groups. In strongly acidic pH (low pH), the amino acids are positively charged (cation) while in strongly alkaline pH (high pH), they are negatively charged (anion). Every amino acid has a characteristic pH at which it carries both positive and negative charges and exists as zwitterions.

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